An electric double layer capacitor receives attention as an electric power storage device capable of easily charging and discharging comparatively high power.
As an electrode of an electric double layer capacitor, a carbon electrode mainly made of a carbon material is used, and activated carbon is known as the carbon material. In the electrode containing the activated carbon as a main component, activated carbon with a large specific surface area is used, because electric charge accumulated in an electric double layer formed on the surface of the activated carbon contributes to the capacitance of the electric double layer capacitor.
Activated carbon is produced by activating a carbon material made from a plant such as sawdust and coconut shell, a carbon material made from pitch or coke derived from coal or oil, or a carbon material made from a synthetic polymer such as a phenol resin.
As an activation method, there are known methods such as a steam-activation method of heating a carbon material to from 500 to 1,000° C. in an oxidizing gas containing carbon dioxide or steam, and an alkaline activation method of mixing a raw carbon material with an alkali metal hydroxide and heat-treating the mixture.
In these days when an electric double layer capacitor is expected to further become widespread, further improvement in its performance has been desired.
An important characteristic of an electric double layer capacitor is, for instance, high energy density. It is necessary for obtaining the high energy density that the activated carbon of an electrode material has high capacitance. As such activated carbon, activated carbon formed by activating (alkaline activating) a carbon material made from pitch or coke derived from coal and oil, or a carbon material made from a synthetic polymer such as a phenol resin with an alkali metal hydroxide is reported (Non-Patent Document 1: “Large Capacitance Capacitor Technology and Materials II” compiled under the supervision of Atsushi Nishino and Katsuhiko Naoi, page 76).
An important characteristic of an electric double layer capacitor includes low internal resistance, as well. It is preferable for the electric double layer capacitor to have lower internal resistance. The electric double layer capacitor using activated carbon produced by an alkaline activation method for an electrode material is also expected to further decrease its internal resistance.
Furthermore, an important characteristic of an electric double layer capacitor includes durability as well. As for the durability, Patent Document 1 (Japanese Patent Laid-Open No. 2003-243265) discloses an electric double layer capacitor which is directed at reducing an amount of a gas generated in a capacitor element when energized, and providing an electric double layer capacitor that shows superior reliability for a long period. It is an electric double layer capacitor formed at the contact surface between an electrolytic solution and a sheet-shaped polarizable electrode in which a polarizable electrode layer containing activated carbon powder, an electroconductive material powder and a binder is integrated with a collector foil made of a metallic foil, wherein the amount of surface functional groups of a carboxyl group, a quinone group, a hydroxyl group and a lactone group, which exist on the surface of activated carbon powder, is controlled to 1,000 μmol/g or less.
In addition, Patent Document 2 (Japanese Patent Laid-Open No. 2001-284188) describes that a capacitor tends to lower its durability, when containing much alkali metal left inside the activated carbon electrode. Activated carbon produced by alkaline activation normally contains much alkali metal left therein, so that in order to remove the metal, the activated carbon is washed with water or hydrochloric acid.
[Patent Document 1]: Japanese Patent Laid-Open No. 2003-243265
[Patent Document 2]: Japanese Patent Laid-Open No. 2001-284188
[Non-Patent Document 1]: “Large Capacitance Capacitor Technology and Material II” compiled under the supervision of Atsushi Nishino and Katsuhiko Naoi, page 76